It is believed the ozone hole over Antarctica is healing despite the slow decline in ozone-killing chemicals still lingering in the atmosphere.

A powerful ozone-depleting chemical is still making its way into the atmosphere from an unknown source decades after it was banned worldwide.

Carbon tetrachloride (CCl4) was once used in dry cleaning and some fire extinguishers before it and other chlorofluorocarbons responsible for the ozone hole over Antarctica became regulated in 1987 under the Montreal Protocol.

All countries that signed that agreement say they had no CCI4 emissions between 2007 and 2012.

But NASA’s Goddard Space Flight Center scientist Qing Liang says that instead of the level of CCl4 disappearing at the predicted rate of 4 percent per year, it’s declining at only 1 percent annually, meaning it is still being released into the atmosphere.

“It is now apparent there are either unidentified industrial leakages, large emissions from contaminated sites or unknown CCl4 sources,” said Liang.